75g FOWLR - running 7 years. no major issues. I do water changes about 15g out every 10 days or so.

Let me run a timeline here. Sorry for the long post but all details should answer any questions or help determine the issues.

Currently in Tank as of last week: 1 hippo tang - 7 years in the tank
yellow tail damsel and black and white damsel - 7 years in the tank
Singapore Angel - been in since April
Chocolate Chip Starfish - couple months

Last week: Did usual PWC 10-15 removed. Replaced with filtered ocean water from Petco ive been using for over a year. Perfect conditions and much more convenient than mixing own water.

Also added 20 pounds of live sand gradually over the week. Diatoms appear by the weekend - Saturday. never had these before

Thursday: Picked out some new additions, Fire Angel and Potters Angel at the LFS. Requested to place them in the same tank for me to pick up Sunday night and have my gf pick out 1 or 2 more to add with it.

- All doing well, B+W damsel chases a chromis for a while, all get along after an hour or two. Fire Angel active nut in the rocks, Potters Angel hides in the rocks. All fish eat except Potters. Psudocromis not doing well, hanging out in the corner.

Monday night: Psudocromis doing worse, Potters angel hiding behind a rock - breathing rapidly. I move the rock around a bit to get it out, so it hangs in the open a little but still in the corner. All eat well except Psudo and Potters do not eat at all. Chromis doing very well, as well as fire angel - no longer timid.

Tuesday night: Potters angel and psudocromis lying on their side at the bottom of the tank breathing rapidly. Die 1 hour later. Starfish gets a hold on the potters angel, I remove both and bring back to the fish store on Wednesday with water sample.

Go to fish store, with dead fish and water sample. They say I have Ammonia 0.1 Nitrte 0 Nitrate .1 They wont give me a refund b/c Ammonia 0.1. Note my home testing I only see 0 and 0.25 is the next color difference. Say I should do a water change ASAP and get this live nitrifying bacteria. Come home an hour later to see b+w damsel dead and Singapore Angel hiding in the rock like the Potter did before it died. Getting REALLY concerned now!

Did 15g water change, made sure to suction out the top layer of sand and diatoms. That sand was dirty! Added double normal dose of Prime - I usually do only 1 capful and added the dose of the denitriflying bacteria as recommended.

This morning, Thursday: Found the Singapore Angel dead. :-( See diatoms sort of coming back.

Tonight - Thursday night: Found 1 of the 5 in the Blue Chromis school lying on the bottom of the tank. Flame angel very active, Hippo Tang not active. Hes a swimmer, just mopey in the back of the tank and looks like he may be next. Comes out a little when he sees me, but does not swim to the top of the tank when I approach it. Hes conditioned to do that because he expects I have food when he sees me. Def not active. :-(

Simply a ton of stuff happening in the tank in a short period of time. New additions will mean more bacteria needs to grow to support them.
Then new sand on top of old changes the type of bacteria that can survive there, what needed oxygen is now in an oxygen free zone...so it's dead. New sand brings in silicates and explains the diatoms, but then sand filtered out due to diatoms being ugly rather than simply burning out.

Simply put, everything in this hobby should go slow or it can easily turn into a situation where you think a nuke went off. I don't recommend more than 1 new fish a week, let alone 7. Screwing with the sandbed didn't help anything either even if no bb was effected, it is stressful to the fish.

if the tank has been going for 7 years there is/was absolutely no need to add anything to boost the bacteria colonies, it would be pointless as all the BB you need is already present, or should be. The BB colonies will grow to a point of maintaining equilibrium in the system, you add more bio-load, the BB need to increase population numbers to adjust, that is why adding too many at one time is risky regardless of the tank size or how long it has been established.

How deep was/is the sand bed and is it fine sand or more coarse?
My thought is if it is a more than 3"-4" deep, then after 7 years it may have gone south on you and is causing a tank crash.

OR
my other thought is you introduced either oodinium (marine velvet) or flukes into the system with one of the new fish. Both can decimate a tank population within days.
one member, Doug, lost almost an entire tank within about a week or so due to eye flukes.